Source map gen apply map

This pull request is based on #46 and #47.

It added a method [SourceMapGenerator.applySourceMap](https://github.com/sokra/source-map/commit/90fd68f047b9226aacff5e712d8139bdff0a33f5).

It allows to merge a SourceMap for a source file into a SourceMapGenerator.

This is step 2 for composing SourceMaps.